sql: joins should support both equality and ON condition

[RaduBerinde]

Right now for JOINs we support three joinPredicates:

• onPrediate: arbitrary ON condition [quadratic]
• crossPredicate : no ON condition (equivalent to onPredicate with always true condition).
• equalityPredicate: equality on pairs of columns condition [linear]

In general, we will need to support both a set of equality columns AND an additional ON condition (e.g. for cases like ON t1.a = t2.a AND t1.x < 2*t2.y)

In light of this, I think that having these separate types is overkill. There should be a single predicate that has an (optional) set of paired "equality" columns and an (optional) ON condition.

CC @knz